In recent years, there has been a fear that fossil fuels including petroleum and coal would run out, and it is urgently required to develop alternative energy sources to these fossil fuels. Accordingly, various methods for nuclear power generation, water power generation, wind power generation, and photovoltaic power generation have been studied, and some have been in actual use.
Photovoltaic power generation, which can convert the energy of sunlight directly into electric energy, has been put to practical use as a new, pollution-free energy source that will be available virtually permanently. Its cost performance in practical use has improved rapidly, making it a promising clean energy source.
Solar batteries used for photovoltaic power generation that directly convert the energy of sunlight into electric energy and serve as the core portion of photovoltaic systems are made of semiconductors such as silicon. They have structures in which photovoltaic elements are arranged in series or in parallel and packaged in various ways into a unit to protect the elements over a long time of 20 years or so. Such a packaged unit, which is called a photovoltaic module, is typically composed of a glass layer to cover the face that receives sunlight, an encapsulant of resin to fill gaps, and a resin sheet called a back sheet for photovoltaic module to protect the backside surface. For the encapsulant, ethylene-vinyl acetate copolymer resins (hereinafter referred to as EVA resins) are often used because they have high transparency and moisture resistance. The back sheet for photovoltaic module requires properties such as mechanical strength, weather resistance, heat resistance, water resistance, chemical resistance, light reflectivity, water vapor barrier properties, adhesion to the encapsulant, design properties, and adhesion of its outermost layer to silicone resin adhesives for attaching a terminal box.
One example of the back sheet for photovoltaic module traditionally used is a back sheet for photovoltaic module including a polyester film and white polyvinyl fluoride films having high weather resistance (e.g., “TEDLAR” (registered trademark) available from Du Pont Kabushiki Kaisha) bonded to both surfaces of the polyester film with an adhesive such as a polyurethane resin. The back sheet for photovoltaic module having such a lamination configuration that a polyester film is sandwiched between the above films has been widely used in the applications of interest. There is also disclosed a configuration in which a polyester film having high weather resistance and gas barrier properties is bonded with an adhesive (JP 2002-026354 A). In general, polyester films, including polyethylene terephthalate resins, have not so high adhesion to EVA resins, which are most commonly used as encapsulant resins. Thus, to improve the adhesive strength, a configuration is proposed in which an adhesion-improving coating layer made of an acrylic resin or an epoxy resin is provided to improve the adhesion to an encapsulant layer made of an EVA resin (JP 2006-332091 A).
General-purpose polyester resin films, when hydrolyzed, have reduced molecular weights and are embrittled, resulting in reduced mechanical properties. Thus, an improvement thereof, i.e., the improvement in moist-heat resistance is demanded. To meet that demand, various studies have been made to prevent the hydrolysis of polyester resins. With regard to biaxially-oriented polyester films, for example, studies have been made to increase the molecular weight of a resin of a film and control the degree of planar orientation to thereby improve the moist-heat resistance (JP 2007-70430 A).
Furthermore, for application to back sheets for photovoltaic module, it is also desired to impart properties other than moist-heat resistance, in particular, UV resistance and light reflectivity to provide more functions. To achieve this, studies have been made to use polyesters made of more than one component or add other components to provide more functions (e.g., JP 2003-155403 A).
To further improve adhesion of a back sheet for photovoltaic module to an encapsulant made of, for example, an EVA resin and the durability of the adhesion, studies have been made to form an adhesive layer made of a fluoro copolymer, an acrylic copolymer, or a polyurethane polymer, which has high durability, and an isocyanate compound on a polyester film (JP 2011-18872 A).
Although providing good initial adhesive strength to the encapsulant, the adhesive layer made of an acrylic resin or an epoxy resin, when exposed to environmental stresses such as heat, humidity, and UV radiation, may undergo a reduction in interfacial adhesion between the adhesion-improving coating layer and the encapsulant layer, which can cause delamination in extreme cases. Furthermore, exposure to light passing through silicon cells may cause photodegradation of the back sheet for photovoltaic module, leading to defects such as yellowing.
Among the properties that the back sheet for photovoltaic module requires, with regard to weather resistance, which is important for stable protection maintained for a long period of time, adding other components (e.g., UV absorbers and inorganic particles) to provide a polyester film, in particular, a polyester film containing ethylene terephthalate units as a major component with more functions disadvantageously promotes degradation due to, for example, hydrolysis during melt mixing in the manufacturing process or during use. Furthermore, although functions of the components added are provided, moist-heat resistance disadvantageously decreases.
Although excellent in adhesive strength to an encapsulant layer and weather resistance of the polyvinyl fluoride films bonded to both surfaces, the above-described back sheet for photovoltaic module having a configuration in which polyvinyl fluoride films are bonded to a polyester film with an adhesive is disadvantageous in that it is expensive and may fail to maintain sufficient protection. For example, delamination may occur due to moist-heat degradation of the adhesive layer, or the polyester film sandwiched between the polyvinyl fluoride films, depending on its properties, may undergo hydrolysis when exposed to heat and humidity for a long period of time to cause a decrease in mechanical strength.
In view of the above disadvantages, it could be helpful to provide a back sheet for photovoltaic modules having adhesive strength to the encapsulant sufficient for the back sheet for photovoltaic module and is excellent in productivity, weather resistance to photoirradiation from the light-receiving side and the rearmost side of a photovoltaic module, and heat-curling properties.
It could also be helpful to provide a back sheet for photovoltaic module having increased adhesive strength to the encapsulant and increased durability of the adhesive strength.